1. Field of the Invention
The present invention relates to a power conversion system and, particularly to a power conversion system for producing a.c. power having the commercial frequency and voltage from a d.c. power source such as a solar power generator, a fuel cell or a battery incorporated in an uninterruptable power supply system.
2. Description of the Prior Art
There is a great demand for power conversion systems for converting d.c. power into a.c. power having the commercial frequency and voltage for use with solar power generators and fuel cells which supply power to existing commercial power systems and for use in uninterruptable power supply systems incorporating batteries. For this purpose, there have been frequently used DC-to-AC converters (inverters) employing the type of pulse width modulation system, and many of them have used a transformer for isolating the d.c. lines from the a.c. lines.
FIG. 1 is a schematic diagram of an inverter for producing a sinusoidal a.c. output by employment of the conventional pulse width modulation system. The arrangement shown includes d.c. input terminals 1 and 2, a.c. power output terminals 3 and 4, transistors 11 through 14, a capacitor 21, a transformer 23, a filter 50, and a d.c. power source 60.
The operation of the above arrangement is as follows. The transistors 11 and 14 in pairs and the transistors 12 and 13 in pairs operate to become conductive or nonconductive alternately and simultaneously. For example, when the transistor pair 11 and 14 becomes conductive, a voltage will appear between the a.c. output terminals 3 and 4 with the former being positive, and when the transistor pair 12 and 13 becomes conductive, a voltage will appear between the terminals 3 and 4 with the latter being positive. Accordingly, by controlling the ON-period of the transistors appropriately, an output voltage having an approximately sinusoidal waveform can be produced at the a.c. output terminals, as shown in FIG. 2. Waveform (a) in FIG. 2 shows the switching operation of the transistor pair 11 and 14, while waveform (b) shows the switching operation of the transistor pair 12 and 13. Waveform (c) shows the a.c. output voltage waveform at the a.c. output terminals 3 and 4 when the filter 50 in FIG. 1 is absent, while waveform (d) shows the a.c. output voltage waveform when the filter 50 is used. Waveform (e) shows the magnetic flux density in the core of the transformer 23 shown in FIG. 1. The foregoing power conversion system is called a sinusoidal inverter of the type employing a pulse width modulation (PWM) system.
In the conventional pulse width modulated sinusoidal inverter, the transistors operate for switching at a frequency incomparably higher than the a.c. output frequency. Nevertheless, the transformer needs to transmit a.c. power at the lower a.c. output frequency, as shown by the variation of the magnetic flux density in the transformer core in waveform (e), and therefore a large low-frequency oriented transformer is required inherently. Such a transformer is bulky and heavy, and comprises the greater part of both the space and the weight of the power conversion system. At the same time, a great proportion of the overall system loss is derived from the loss created by the transformer.